Thermistor (NTC) Symbol
Definition: The Thermistor (NTC) symbol represents a thermally-sensitive resistor whose resistance decreases as temperature rises (Negative Temperature Coefficient), drawn in circuit diagrams as a standard resistor rectangle (IEC 60617) or zigzag (ANSI/IEEE 315) with a diagonal arrow and the letter 't' or 'θ' overlay, indicating temperature-dependent variation, and assigned the designator R or RT.
Also known as: NTC thermistor, negative temperature coefficient resistor, thermal resistor, NTC sensor, temperature-dependent resistor, thermally-sensitive resistor.
What the Thermistor (NTC) symbol means
The Thermistor (NTC) symbol denotes a passive, two-terminal semiconductor device whose resistance has a strong, predictable negative relationship with temperature. Unlike a fixed resistor, the thermistor's resistance can change by orders of magnitude across its operating range, making it highly sensitive to small temperature changes and ideal as a temperature sensor.
In circuit diagrams, the NTC thermistor symbol (designator R or RT) appears wherever a circuit must sense, measure, or compensate for temperature. The NTC variant is by far the most common thermistor type used in temperature measurement and inrush-current limiting. Its complement, the PTC (Positive Temperature Coefficient) thermistor, uses the same base symbol with a different annotation, and its resistance increases with temperature.
How to identify the Thermistor (NTC) symbol
The thermistor symbol is drawn as a resistor rectangle (IEC 60617) or zigzag (ANSI/IEEE 315) with a superimposed or adjacent lowercase 't' (for temperature) or the Greek letter theta (θ), often accompanied by a diagonal arrow through the body to indicate a variable parameter. In IEC 60617 the temperature-dependent resistor is specifically shown as a rectangle with a '+t°' label for PTC or '−t°' label for NTC, making the coefficient direction explicit. Terminals are labelled A and B (or simply the two wire leads).
Function in a circuit
The NTC thermistor operates as a variable resistor controlled by temperature. As the thermistor's temperature rises, the number of charge carriers in its semiconductor material increases, lowering resistance according to a steep exponential relationship described by the Steinhart-Hart equation or the simplified B-parameter equation (R = R₀ × e^(B×(1/T − 1/T₀))). This large resistance swing per degree of temperature change is exploited to measure temperature accurately in the −55 °C to +150 °C range, to compensate for temperature-induced drift in other circuit components, and in power supplies to limit inrush current at turn-on (the thermistor starts cold and resistive, then heats up and drops resistance).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 database (symbol 04-02-06 and related entries) shows the thermistor as a rectangle with a temperature coefficient indicator. The qualifying symbol '−t°' inside or beside the rectangle denotes NTC; '+t°' denotes PTC. IEC 60539 series covers fixed solid-state thermistors specifications. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 shows the thermistor as a zigzag resistor body with a diagonal arrow and the letter t or the temperature symbol. The designator is RT (resistor, temperature-sensitive) or simply R with a subscript T. |
| Key difference | IEC uses a filled rectangle body with an explicit '−t°' NTC annotation inside. ANSI uses the zigzag body with a diagonal arrow. Both convey the same meaning; the IEC annotation makes the coefficient polarity (NTC vs PTC) visually explicit. |
Terminals / pins
| Pin | Name |
|---|---|
| a | A |
| b | B |
Typical values
Resistance at 25 °C (R₂₅): commonly 1 kΩ, 2.2 kΩ, 4.7 kΩ, 10 kΩ, 47 kΩ, 100 kΩ. B-parameter (sensitivity constant): typically 2000–5000 K. Temperature range: −55 °C to +150 °C (standard NTC). Tolerance: ±1 % to ±5 % on R₂₅. Power rating: 0.1 W to 1 W typical for small bead/disc types.
Where the Thermistor (NTC) symbol is used
- Temperature measurement in microcontroller circuits (voltage-divider bridge with the NTC and a known resistor feeding an ADC pin)
- Battery temperature monitoring in Li-ion charger ICs to prevent thermal runaway during charging
- Inrush current limiting in AC power supply inputs (series NTC thermistor drops resistance after warm-up)
- HVAC thermostat and temperature-controller sensor inputs for room and duct temperature sensing
- Medical devices — thermistors used in digital thermometers, incubators, and blood-temperature monitors
- Automotive coolant and exhaust temperature sensing for engine management systems
Example
In a microcontroller temperature-sensing circuit, the NTC thermistor (10 kΩ at 25 °C, B = 3950 K) is connected in a voltage-divider between VCC (3.3 V) and GND with a 10 kΩ fixed resistor; the mid-point voltage is fed to an ADC input on pin A0 of a microcontroller, which computes the temperature using the Steinhart-Hart coefficients or a lookup table.
Key facts
- The Thermistor (NTC) symbol is a resistor rectangle (IEC) or zigzag (ANSI) with a '−t°' annotation or diagonal arrow, indicating resistance decreases as temperature increases.
- NTC thermistors follow an exponential resistance-temperature relationship approximated by the B-parameter equation: R = R₀ × e^(B×(1/T − 1/T₀)), where T is temperature in Kelvin.
- The standard designator for a thermistor is R or RT; the NTC type is distinguished by the '−t°' label (IEC) or a negative-slope annotation (ANSI).
- The thermistor has two terminals: A and B (no polarity for passive NTC devices — the device is non-polarised and can be connected in either orientation).
- NTC thermistors are significantly more sensitive than RTDs (resistance temperature detectors) over small temperature spans, but less linear over wide spans; they are characterised by their R₂₅ value (resistance at 25 °C) and B-parameter.
- IEC 60539-1 specifies standard NTC thermistor characteristics; IEC 60617 governs the schematic symbol.
- PTC thermistors use the same base symbol with a '+t°' annotation and behave oppositely — resistance increases steeply above the Curie temperature, used for self-resetting overcurrent protection (polyfuses).
- In power supply inrush-current limiting applications, the NTC thermistor is placed in series with the AC mains input; its cold resistance (at room temperature) limits the initial capacitor-charging surge, then its resistance drops as it self-heats in normal operation.
Diagrams that use this symbol
- coolant temperature sensor wiring diagram
- 2 wire temp sensor coolant temperature sensor wiring diagram
Frequently asked questions
What does the thermistor symbol look like in a circuit diagram?
The NTC thermistor symbol looks like a standard resistor (a rectangle in IEC 60617 schematics, or a zigzag in ANSI/IEEE 315 schematics) with an additional temperature annotation: a '−t°' label inside or beside the rectangle (IEC) or a diagonal arrow and the letter 't' through the zigzag body (ANSI). The annotation indicates the resistance varies with temperature, and the minus sign or NTC label specifies the negative coefficient.
What does NTC mean in a thermistor?
NTC stands for Negative Temperature Coefficient. An NTC thermistor's resistance decreases as temperature increases. This is the opposite of a PTC (Positive Temperature Coefficient) thermistor, whose resistance increases with temperature. NTC thermistors are the most common type used for temperature measurement because their large resistance change per degree provides high sensitivity.
What is the difference between a thermistor symbol and a resistor symbol?
A standard fixed-resistor symbol (IEC rectangle or ANSI zigzag) has no annotation. The thermistor symbol adds a temperature-sensitivity indicator: in IEC 60617 this is a '−t°' (NTC) or '+t°' (PTC) label inside the rectangle; in ANSI/IEEE 315 it is a diagonal arrow through the body plus the letter 't'. The diagonal arrow indicates a variable (temperature-controlled) parameter.
What is the designator letter for a thermistor?
The standard designator for a thermistor is RT (R for resistor, T for temperature-sensitive) per ANSI/IEEE 315-1975, or simply R with a subscript T. In practice, many schematics use NTC1, NTC2 or TH1, TH2 as reference designators. IEC schematics may label the device R with a temperature coefficient annotation.
What are typical resistance values for an NTC thermistor?
Common NTC thermistor resistance values at 25 °C (R₂₅) are 1 kΩ, 2.2 kΩ, 4.7 kΩ, 10 kΩ (the most popular value for microcontroller circuits), 47 kΩ, and 100 kΩ. The B-parameter (sensitivity constant) is typically between 2000 K and 5000 K. Resistance typically drops to a fraction of the 25 °C value at 100 °C.
What standard defines the thermistor schematic symbol?
The thermistor schematic symbol is defined in IEC 60617 (the IEC graphical symbols for diagrams database) for IEC-convention schematics, and in ANSI/IEEE 315-1975 for North American schematics. The physical and electrical characteristics of NTC thermistors are governed by IEC 60539-1 (fixed solid-state thermistors for direct-heated NTC types).
Is an NTC thermistor polarised?
No. A standard passive NTC thermistor is a non-polarised two-terminal device; it can be connected in either orientation without affecting operation. It has no anode or cathode. Terminals are conventionally labelled A and B.
What is the difference between a thermistor and an RTD?
An NTC thermistor has a much larger resistance change per degree than an RTD (Resistance Temperature Detector), giving higher sensitivity, but its response is highly non-linear (exponential). An RTD (such as a Pt100 or Pt1000) has a near-linear resistance-temperature response and is more accurate over wide temperature ranges but requires more signal-conditioning circuitry. Thermistors are preferred for narrow-range, high-sensitivity applications; RTDs for precision wide-range measurement.
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